Object created by joining several objects doesn't rotate uniformly consistently
Your problem is with the precision of your model, you should improve your modeling skills and take care of your mesh.

You model needs some serious clean up, there are quite a few misplaced vertex, inverted normals, non manifold faces, overlapping geometry, etc.

There is also one unused Copy Rotation constraint, probably a leftover from when there were several objects

Now to the problem at hand, what you are seeing is probably an optical illusion, your model seems to be rotating at different speeds because the center is not accurately placed relative to the geometry, making the center part "wobble" in relation to the rotation center.



Now your geometry is really uneven and you should probably model it cleanly from scratch, but if you don't want to, just enter edit mode, select the central vertex, move the cursor to selected, exit edit mode and align objects center to the cursor position.



Do this for all objects, although they are all slightly skewed and wont easily be totally perfectly aligned